Naturally Occurring Asbestos: the problem of quantification

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Comminution effects on mineral grade distribution

Every mining operations is followed by a beneficiation process aimed to deliver quality material to the transformation industry. Most of mineral processing is focused on comminution and grinding of extracted ore that is crucial for the following separation steps in order to divide gangue from valuable minerals. Comminution is the most energy consuming phase and the quality of the results is strictly related to the characteristic of the material under treatment. A preliminary study has been performed to understand the crushing behaviour of a mixed sulphide ore, containing galena and sphalerite, and how target minerals grade distributes among the different sized products of the process. Ore samples have been examined and characterized by means of thin sections observation and SEM analyses for the determination of the grain liberation size, while XRD quantitative analyses have been performed for the definition of the grades. The crushing circuit selected comprises lab-scale impact crusher, jaw crusher, disk mill and rod mill. The products collected are below the free grain size threshold and granulometric analyses have been performed. Comminution products have been divided in size classes suitable for flotation separation, ranging between 0.250 and 0.075mm and XRD analyses showed a variable mineral grade distribution varying with the reduction in dimension of the products. This important trend should be considered for further investigation related to an efficient froth flotation separation

Evaluation of errors in quantitative determination of asbestos in rock

The quantitative determination of the content of asbestos in rock matrices is a complex operation which is susceptible to important errors. The principal methodologies for the analysis are Scanning Electron Microscopy (SEM) and Phase Contrast Optical Microscopy (PCOM). Despite the PCOM resolution is inferior to that of SEM, PCOM analysis has several advantages, including more representativity of the analyzed sample, more effective recognition of chrysotile and a lower cost. The DIATI LAA internal methodology for the analysis in PCOM is based on a mild grinding of a rock sample, its subdivision in 5-6 grain size classes smaller than 2 mm and a subsequent microscopic analysis of a portion of each class. The PCOM is based on the optical properties of asbestos and of the liquids with note refractive index in which the particles in analysis are immersed. The error evaluation in the analysis of rock samples, contrary to the analysis of airborne filters, cannot be based on a statistical distribution. In fact for airborne filters a binomial distribution (Poisson), which theoretically defines the variation in the count of fibers resulting from the observation of analysis fields, chosen randomly on the filter, can be applied. The analysis in rock matrices instead cannot lean on any statistical distribution because the most important object of the analysis is the size of the of asbestiform fibers and bundles of fibers observed and the resulting relationship between the weights of the fibrous component compared to the one granular. The error evaluation generally provided by public and private institutions varies between 50 and 150 percent, but there are not, however, specific studies that discuss the origin of the error or that link it to the asbestos content. Our work aims to provide a reliable estimation of the error in relation to the applied methodologies and to the total content of asbestos, especially for the values close to the legal limits. The error assessments must be made through the repetition of the same analysis on the same sample to try to estimate the error on the representativeness of the sample and the error related to the sensitivity of the operator, in order to provide a sufficiently reliable uncertainty of the method. We used about 30 natural rock samples with different asbestos content, performing 3 analysis on each sample to obtain a trend sufficiently representative of the percentage. Furthermore we made on one chosen sample 10 repetition of the analysis to try to define more specifically the error of the methodology

Comminution Effects on Mineral-Grade Distribution: The Case of an MVT Lead-Zinc Ore Deposit

Every mining operation is followed by a beneficiation process aimed at delivering quality materials to the transformation industry. Mainly, in order to separate valuable minerals from gangue in mineral processing, the crushing and grinding of extracted ore are crucial operations for the following separation steps. Comminution is the most energy-consuming operation in mining, and the quality of the results is strictly related to the characteristic of the material under treatment, the type of equipment used in comminution, and the circuit design adopted. A preliminary study was performed in order to understand the crushing behavior under different comminution forces of a high-grade mixed Zn-Pb sulfide ore sample, collected in a Mississippi-Valley Type (MVT) deposit, and the distribution of the target minerals among the products of the process. Ore samples were examined and characterized through thin section observation and SEM analyses for the determination of grain size and texture features, while X-ray powder diffraction (XRPD) quantitative analyses were performed for the definition of target mineral concentrations of comminuted product samples. The selected crushing and grinding circuit comprised lab-scale equipment. For each stage of the process, products below the estimated free-grain size threshold were collected, and particle size analyses were carried out. Comminution products were divided into size distribution classes suitable for further separation operations, and XRPD analyses showed a mineral-grade distribution varying with the dimensions of the products. Characterization of the ore material after crushing and grinding force applications in terms of the distribution of target minerals among different-sized classes was achieved. The important trends highlighted should be considered for further investigation related to an efficient separation

CIRCULAR ECONOMY IN THE EXTRACTIVE SECTOR

The mining and the quarry sector shares the need to enhance the by-products as one of the most effective tools to limit waste production. This has the aim of minimizing the environmental cost of raw materials upstream production chains and to support research and technological innovation, providing opportunities for growth, competitiveness and creation of added value for important sectors. The exploitation of mines, quarries and the related treatment plants produces large quantities of "tailings". The by-product can be reused for different purposes within the construction supply chain (construction of road embankments, concrete, pre-mixed products, fillers etc.) thanks to their shape, size, physical and petrographic characteristics. For this reason it is essential to enhance the use of by-products of the extractive industry according to the last European regulation and implementing the National ones. In particular, the main crucial points in extractive industry to be faced and improved applying the Circular Economy approach are: - mining/quarry tailings. - sawing sludge from ornamental stone industry. - waste water from aggregate processing. The “circular approach” in the extractive industry is possible considering by-products in zero-waste supply chains as valuable secondary raw materials since the very beginning of every operation planning. A change in this vision is needed to sustain, also by means of proper indicator, an even more efficient circular economy

Grinding Test on Tremolite with Fibrous and Prismatic Habit

The main objective of this work is the evaluation of the morphology change in tremolite particles before and after a grinding process. The crushing action simulates anthropic alteration of the rock, such as excavation in rocks containing tremolite during a tunneling operation. The crystallization habit of these amphibolic minerals can exert hazardous eects on humans. The investigated amphibolic minerals are four tremolite samples, from the Piedmont and Aosta Valley regions, with dierent crystallization habits. The habits can be described as asbestiform (fibrous) for longer and thinner fibers and non-asbestiform (prismatic) for prismatic fragments, also known as “cleavage” fragments. In order to identify the morphological variation before and after the grinding, both a phase contrast optical microscope (PCOM) and a scanning electron microscope (SEM) were used. The identification procedure for fibrous and prismatic elements is related to a dimensional parameter (length–diameter ratio) defined by the Health and Safety Executive. The results highlight how mineral comminution leads to a rise of prismatic fragments and, therefore, to a potentially safer situation for worker and inhabitants

Identification of dangerous fibers: some examples in Northern Italy

The presence of asbestiform minerals has to be foreseen in the planning of infrastructural activities: Asbestos can be a component of sedimentary rocks or of mafic and ultra mafic metamorphic rocks. Surveys and core drilling, in addition to providing important information on the quality of the rock and its geotechnical characteristics, allow for a prediction of the presence of asbestiform minerals in the areas affected by mining or infrastructural activities. During the excavation, workers can be exposed to the asbestos risk, therefore, the control of the air quality and of the excavated materials are fundamental for the safety of involved people. In this work some problems we met in the analysis of airborne filters and bulk samples from sites in northern Italy are presented. The asbestos fibers present in rocks as accessory minerals, are often different in habit and dimension from the well-known asbestos fibers used as industrial minerals and moreover can be erroneously identified as minerals morphologically and chemically similar present in the same rock or environment. In the case of tunnel muck it could be contaminated by substances used for the excavation that could modify colours and optical properties of asbestos minerals. In the PCOM (Phase Contrast Optical Microscope) analysis chrysotile, sepiolite and antigorite, due to their different refraction index, when the fibers have dimension > 0,5 micron and aren’t contaminated by lubricant can be easely identified even if the morphology of chrysotile is very similar to that of sepiolite. In Electron Scanning Microscope (SEM) the discrimination between chrysotile and antigorite on the airborne filters is not always possible because the fibers of thin dimensions show similar habit and spectrum. In the case of the tremolite amphibole, morphology changes from prismatic to fibrous depending on its origin (p.eg. Monastero, Val Grana, Verrayes, Brachiello). Both prismatic and asbestiform tremolite (Gamble and Gibbs, 2007; Addison and McConnel, 2007) may show inhalable elements with width less than 3 micron, length more than 5 micron and width length ratio 1:3, whose dangerousness (fiber coming from fibrous tremolite or the cleavage fragments coming from prismatic tremolite) could be different and it is object of epidemiologic studies